master vein, n. — Green’s Dictionary of Slang

master vein n.

[SE master-vein, a major vein, usu. the carotid artery or jugular vein]

In phrases

hit on the master vein (v.)

to become pregnant.

1592

Greene Disputation Betweene a Hee and a Shee Conny-Catcher (1923) 29: My faire daughter was hit on the master vaine and gotten with childe.

prick the master vein (v.)

of a man, to have sexual intercourse (cf. hit on the master vein ).

1672

‘The Northern Ladd’ in Ebsworth Roxburghe Ballads (1893) VII:1 171: A Barber-Surgeon came to me, whom I did take in great disdain, / He said his art I soon should see, / for he would prick my master-vein.